This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / The Amazing Maurice and His Educated Rodents
Near the very end: "I'd hoped there was an island somewhere," said Dangerous Beans. "A place where rats could really be rats." This made me sob for five minutes, in spite of the generally happy conclusion of the book.
When Maurice and Dangerous Beans die. This troper bawled like a little kid, hit very hard with the fact that life was real, cruel, and didn't get storybook happy endings, for several minutes straight.
The discovery of what's staining the trickle of water on the cellar floor red. It's the Mr. Bunnsy book, abandoned in a puddle because Peaches and Dangerous Beans have lost faith in it.